Insecticidally active pyrethroids, e.g. cinerin, allethrin, phthaltrin, permetrin, cypermetrin, decametrin, are generally synthesized starting from a lower aliphatic ester of a corresponding cyclopropanecarboxylic acid. Pyrethroids can conveniently be prepared by hydrolyzing these esters into the corresponding free acids and coupling the acids obtained with the alcohol components of the desired active compounds by conventional techniques of esterification.
The conventionally employed hydrolysis methods have numerous disadvantages.
Alkaline hydrolysis of dihalovinylcyclopropanecarboxylic acid esters results in the formation of haloacetylenecyclopropanecarboxylic acid by-products, which are very difficult to eliminate. Therefore these methods are practically not suitable to prepare the desired active pyrethroid compound in a pure form. Attempts have been made to perform hydrolysis under acidic conditions but the purity of the products was not satisfactory and therefore they had to be recrystallized.
By the known methods mixtures of cis- and trans-cyclopropanecarboxylic acid esters are produced. (Coll. Czech. Commun. 242230 (1959)). The activity of pyrethroid compounds prepared from the two isomers is considerably different. It may therefore be desirable to separate the isomers. The methods known in the art generally relate to the separation of the cis- and trans-free acids, preferably by recrystallization from suitable solvents (J.Chem.Soc. 283, 1945).